1. Field of the Invention
This invention pertains to arc welding, and more particularly to apparatus for controlling the voltage at a welding arc.
2. Description of the Prior Art
Many welding jobs are located at a distance from a central source of electric power. To perform those jobs, self-contained welding machines have been developed. For example, welding machines that include an internal combustion engine and a generator are well known and are in widespread use.
Engine-driven welding machines are often transported to a welding site by a motor vehicle, such as a truck. Many trucks large enough to transport an engine-driven welding machine have built-in hydraulic systems, including a hydraulic pump, for operating such accessories as lifts, cranes, and baskets. For convenience, the welding machine normally remains on the truck during welding operations. Thus, two internal combustion engines are present at the job site: the engine in the truck, and the one in the welding machine. Obviously, the truck hydraulic system is also present at the job site.
To improve the economics associated with truck transported welding machines, it is known to use a hydraulic motor to drive a generator to produce electrical energy. At the job site, the welding machine hydraulic circuit is connected to the truck hydraulic pump. The truck engine, acting through the truck hydraulic pump and the welding machine hydraulic motor, thus becomes the prime mover for the welding machine generator.
Prior hydraulic driven welding machines have not proven entirely successful. The major problem was that the truck engine could not drive the truck hydraulic pump, and ultimately the welding machine generator, at a constant speed such that the generator produced a constant electrical output. The problem was especially severe at the start of a welding operation when the arc was struck. The initial surge of current loaded the generator and slowed it down to the point where the arc could not be maintained. For example, it was not uncommon for a generator that was rotating at 3600 revolutions per minute at no load to slow to as little as 2500 revolutions per minute at the start of high weld output level. That speed was too low to maintain a proper welding voltage. Even if the welding machine operator was successful in striking and maintaining an arc, the generator speed tended to pulsate causing the current to vary during welding. The corresponding variation in the welding voltage made it very difficult to produce a quality weld. Conversely, at the end of a heavy welding operation, the generator speed would increase dramatically, to as much as 4000 or even 4500 revolutions per minute when the arc was extinguished.
In an attempt to solve the generator speed problem, a person initially set the prime mover speed, i.e., the truck engine speed, via a manual or automatic throttle control. However, as the system heated, the fluid viscosity changed, possibly changing the generator/welder speed. When that occurred, the person was required to return to the vehicle to reset the prime mover speed.
Another proposed solution to the generator regulation problem was to add a governor to the truck engine. Although the governor was an improvement over manual engine speed adjusting, it nevertheless was not completely satisfactory. One reason was that even with a relatively constant engine speed, the flow of the hydraulic fluid from the truck pump to the welding machine motor could vary due to viscosity changes. At start up, for example, the hydraulic fluid was cool. As the system warmed during use, the fluid viscosity decreased and therefore tended to change the hydraulic motor speed despite the constant engine speed.
A further potential problem in prior hydraulic welding machines concerned the hydraulic pumps. The pumps frequently had performance characteristics that changed with changing system operating conditions. The pump therefore introduced another variable into the hydraulic welding machine.
Thus, a need exists for increased performance of the hydraulic systems of hydraulic welding machines.